A hands free system permits access to a vehicle and starting it without having to use a mechanical key. The user of the vehicle is thus simply provided with an electronic card (also called in what follows a badge) which is detected and recognized by a control and management device associated with antennas disposed onboard the vehicle. If the badge is identified by the control and management device as being a badge permitted to the vehicle, the bearer of this badge can enter the vehicle simply by using a door handle and start the motor of the vehicle by simple action on a button.
Generally, for such a hands free system, a group of antennas is adapted to detect the presence of the badge outside the vehicle and another within the vehicle. If an authorized badge is detected outside the vehicle, the control and management system will permit access to the vehicle without however permitting starting, whilst if the badge is within the vehicle, a starting of the motor could take place.
To locate the badge, a signal is sent to a group of antennas which carry out a corresponding emission, whilst no signal is sent to the other antennas so that they remain inactive. The antennas used are LF (low frequency) antennas emitting a magnetic field of a frequency conventionally about 125 kHz with a restricted range thereby permitting carrying out good location of the corresponding badge. The dialogues between the internal antennas and the badge are of a different nature from the dialogues between the external antennas and the badge. This permits the badge to recognize in advance the response desired by the vehicle. This response is generally effected by emission of an RF (radio frequency) signal of a frequency generally of about 433 MHz.
Each antenna is electrically similar to a coil and is positioned in the vehicle as a function of the desired emission zone. An electronic device permitting generating a signal, modulating it and sending it to the coil is associated with the antenna. This electronic device is generally integrated into the control and management computer provided in vehicles for the electronic management of the interior of the passenger compartment of the vehicle. The antenna is thus connected to the computer by means of a cable and connectors.
The problem which thus arises is to be sure that the antenna is well connected to the computer. Thus, a branched antenna is difficult to detect and of course causes several anomalies of operation of the hands free system.